[ena] Increase receive ring size to 128 entries

[ena] Increase receive ring size to 128 entries

Some versions of the ENA hardware (observed on a c6i.large instance in
eu-west-2) seem to require a receive ring containing at least 128
entries: any smaller ring will never see receive completions or will
stall after the first few completions.

Increase the receive ring size to 128 entries (determined empirically)
for compatibility with these hardware versions.  Limit the receive
ring fill level to 16 (as at present) to avoid consuming more memory
than will typically be available in the internal heap.

Signed-off-by: Michael Brown <mcb30@ipxe.org>